A rubber belt or a chain is used for driving a camshaft of an internal combustion engine of an automobile, driving an auxiliary unit such as an injection pump, or power transmission in an industrial machine. Generally, a rubber belt includes a rubber portion and a reinforcing cord embedded in the rubber portion. Since the strength of the rubber belt depends on the strength of the reinforcing cord, the reinforcing cord is an important component that determines the life of the rubber belt. Typically, the reinforcing cord includes reinforcing fibers and a coating layer formed on the surface of the fibers. As examples of such a reinforcing cord, rubber belts in which carbon fibers are used as reinforcing fibers have been proposed (for example, JP 2004-225178 A and JP 2010-024564 A).
A reinforcing cord using carbon fibers has a drawback that the tensile strength of the cord decreases as the belt moves. One of the causes may be that the fibers in the reinforcing cord are locally broken due to local high stress concentration thereon during use, and thus the strength of the reinforcing cord decreases. Another cause may be the deterioration of the coating layer of the reinforcing cord. As the coating layer deteriorates, the effect of protecting the fibers decreases, and the integrity, flexibility, etc. of the cord decrease accordingly.
The elongation at break of many of glass fiber cords used as reinforcing cords is about 4%. On the other hand, the elongation at break of carbon fiber cords as high-modulus fiber cords is as low as about 2%. Therefore, the fact that brittle and high-modulus fibers such as carbon fibers have such a low elongation at break should be kept in mind when they are used in a reinforcing cord. That is, when the reinforcing cord is subjected to a given bending deformation, it should be protected from overload that would result in breakage of carbon fibers. If the coating layer formed on the reinforcing fibers is hard, the load on the reinforcing fibers may increase when the reinforcing cord is subjected to bending deformation.
On the other hand, as one of the techniques for obtaining a carbon fiber cord which is flexible for bending deformation, it may be possible to increase the number of twists of the cord. However, since the elastic modulus of the cord decreases if the number of twists is increased, such a cord cannot be used for applications that require high-modulus belts.